The present invention relates to a wind turbine.
Modern wind turbines are divided into two major categories: horizontal axis turbines and vertical axis turbines. Horizontal axis wind turbines typically comprise a tower and a fan-like rotor mounted at the top of the tower for rotation about an axis substantially parallel to the earth's surface. The rotor of a horizontal axis wind turbine must face either into or away from the direction of the wind and a yaw mechanism is required to rotate the rotor about the vertical axis of the tower to keep the rotor in proper alignment with the wind flow. Since a mechanical means of delivering power to the ground could cause the rotor to yaw out of alignment with the wind, energy conversion devices, such as generators; power transmission equipment; and related equipment are typically also mounted atop the tower. A structurally robust and costly tower is required to support the weight of the elevated equipment. In addition, the tower structure must be resist oscillation and fatigue resulting from pressure pulsations produced by the interaction of the moving rotor blades and the tower. Likewise, the pressure pulse created by the wind shading of the tower causes the blades of the rotor to flex inducing fatigue in the blades and other rotor components. Maintenance of horizontal axis turbines can be complex because the equipment is located at the top of the tower. A large crane is typically required to replace equipment or to support the rotor during bearing replacement or maintenance. While horizontal axis wind turbine installations are relatively complex and expensive, they are the most common wind turbine configurations in current use.
Vertical axis wind turbines comprise, generally, a central shaft arranged vertically with respect to the ground and rotatably supporting a plurality of blades or vanes arrayed around the shaft and roughly perpendicular to the wind flow. Vertical axis turbines do not require a yaw mechanism to align the blades with the wind and the generator or other energy converter and related power transmission equipment may be mounted on the ground at the base of the turbine, potentially substantially reducing the complexity and cost of the installation.
Vertical axis wind turbines are divided generally into lift- and drag-types. Drag-type vertical axis wind turbines, exemplified by the three-cup anemometer and the Savonius wind turbine, are rotated by the force produced by the wind impinging on the exposed area of cups, buckets, or paddles arranged around a vertical shaft. Savonius, U.S. Pat. No. 1,697,574, incorporated herein by reference, discloses a vertical axis wind turbine that can be described as a barrel cut in half lengthwise with the halves offset to form two scoops and mounted on a vertical shaft. The efficiency of a Savonius turbine is limited because power produced by the gathering side of the rotor is offset by drag produced by the other side of the rotor. In addition, since the area of the scoops exposed to the wind flow varies as the turbine rotates, the torque is not even throughout a revolution of the shaft and no torque will be produced to initiate rotation if the rotor is improperly aligned with the wind flow. Further, the maximum velocity of the cups or paddles of a drag-type turbine is substantially equal to the velocity of the wind (tip speed ratio≈1). While this type of turbine can produce high torque and can be useful for pumping water and similar tasks, the speed of rotation is generally too slow for efficient production of electricity, a major use of commercial wind turbines.
Lift-type vertical axis turbines rely on the lift force generated as the wind flows over an air foil to obtain tip speeds exceeding the wind's velocity. Darrieus, U.S. Pat. No. 1,835,018, incorporated herein by reference, discloses a wind turbine typifying lift-type vertical axis wind turbines. The Darrieus wind turbine is the only vertical axis wind turbine ever manufactured commercially in any volume. The Darrieus wind turbine may comprise C-shaped rotor blades attached at their top and bottom ends to a vertical central shaft or rectilinear blades arranged parallel to the shaft in a cylindrical drum or squirrel cage arrangement (sometimes referred to as a “Giromill”). Darrieus turbines typically have two or three blades. Since lift forces provide the torque for rotation, the tip speed of the blades can exceed the speed of the wind. Darrieus wind turbines can have a tip speed ratio exceeding three making this type of turbine suitable for electric power generation.
While vertical axis wind turbine installations are potentially less complex and costly than horizontal axis turbines, the lack of commercial success of vertical axis turbines is indicative of substantial drawbacks of this type of turbine. Since no tower is required, a major cost of a wind turbine installation is eliminated. However, wind speeds close to the ground are very low and turbulent due to boundary layer effects. As a result, the output of a vertical axis turbine, particularly the lower half of the rotor, is limited and the overall efficiency is relatively low. Further, guy wires may be required to stabilize the vertical shaft which may make the turbine impractical in extensively farmed or built-up areas. While the power conversion equipment can be mounted at ground level, a crane is typically required to lift the vertical shaft and blades for bearing replacement or maintenance. In addition, lift-type vertical axis turbines are not self starting, but an electric generator connected into a power grid can be used as a motor to start the turbine.
What is desired, therefore, is a wind turbine combining the lower cost and reduced complexity of a vertical axis wind turbine with the higher efficiency and performance of a horizontal axis wind turbine.